The time course of changes in contraction kinetics during the tonic activation of the rat tracheal smooth muscle.

The tension increase after onset of electrical stimulation (30 Hz square wave; 1.65 ms pulse duration) and after the cessation of inhibiting length vibration (frequency 100 Hz sinus; amplitude 6% of the muscle length) was analysed in the isolated rat tracheal smooth muscle. In the first experimental series, tonic contraction was interrupted by a 2 s vibration applied 4-256 s after a preceding stimulation. In the second series, the onset of force development was delayed by a long-term vibration stopping 6-258 s after the commencement of a simultaneously performed electrical stimulation. In both the experimental series tested, the time course of post-vibration tension recovery showed an initial fast and a subsequent slow component. The former reflects the kinetics of cross-bridge reattachment and the latter those of the normal actin-myosin interaction cycle. The time constants of both these components reached a minimum of 0.58 +/- 0.04 s (fast) and 3.49 +/- 0.28 s (slow component) when the vibration stopped 18 s after the start of stimulation. Both values increased to 1.29 +/- 0.15 s and 9.98 +/- 1.17 s after a preceding stimulation of 256 s. These changes in the time constants may reflect the slowing of cross-bridge action under prolonged contraction. Such variations in the time constants of post-vibration tension recovery occurred without any corresponding changes in the steady state tension developed after cessation of vibration. These results lend further support to the supposition that different mechanisms might control the rate and extent of smooth muscle contraction.